Controlled thermal coating

ABSTRACT

The combined measurement of the particle velocity, temperature, intensity, and burner voltage and control thereof in a tolerance range makes it possible to keep the layer structure, the layer thickness and the layer weight constant in spite of wear-induced variations in the coating process.

The invention relates to a thermal coating process. Thermal spraying processes are used for producing metallic and ceramic layers, in which a material melts completely or at least partially.

The material is injected into a nozzle, for example of a plasma torch, or externally. The nozzle, at least, becomes worn owing to very high plasma temperatures and the inflow of powder material. This leads to wear-related fluctuations in the coating process, which are caused primarily by a drop in voltage at the torch.

To date, these fluctuations have been compensated for by readjusting the powder mass flow, in order to keep the desired layer weight of the blade or vane in the tolerance range.

This is not optimal, however, since merely the drop in power at the torch which is induced by the drop in voltage is compensated for by an increase in the powder mass flow.

It is an object of the invention, therefore, to solve the aforementioned problem.

The object is achieved by a method as claimed in claim 1.

The dependent claims list further advantageous measures which can be combined with one another, as desired, in order to obtain further advantages.

In the drawings:

FIGS. 1-3 show parameter profiles from the prior art,

FIGS. 4-9 show parameter profiles according to the invention,

FIG. 10 shows a nozzle,

FIG. 11 shows a turbine blade or vane.

The description and the figures represent only exemplary embodiments of the invention.

Coatings are applied by thermal coating processes such as SPPS, HVOF, APS, LPPS, VPS, . . . . In these processes, a plasma or a flame is generated in a nozzle, a material flowing in through the nozzle or at the end of the nozzle.

The wear on the nozzle or on the coating apparatus causes the material flow properties and therefore also the degree of melting of the material, in particular of the powder, to change.

FIG. 1 shows an exemplary profile of the voltage U_(B) between the nozzle 30 and an electrode 36 (FIG. 10) according to the prior art.

The voltage U_(B) between the nozzle 30 and the electrode drops over time t and then merges into saturation. In the case of other types of nozzle, a continuous drop in the voltage U_(B) over time t or other profiles are also possible.

The profile of the average temperatures T and of the average material flow velocity v_(p) (not shown) over time is analogous.

As an effect of this, the layer weight m_(c) decreases over time (FIG. 2) and/or the porosity p (FIG. 3) increases.

The properties of the flame or of the plasma and/or of the molten material, which emerge from the nozzle 30 during the thermal coating, in particular during the plasma coating or HVOF coating, are therefore determined according to the invention.

In this respect, what are determined are target values Z1, Z2, Z3, such as in particular of the voltage U_(B) between the nozzle 30 and the electrode 36, material flow velocity v_(p), temperature T of the material flow 42

This is effected by measuring instruments, which determine quantitative data by way of pyrometry or CCD cameras.

If deviations are thus identified during the measurement, it can be concluded that wear has occurred, and parameters R1, R2, R3 for varying the target variables Z1, Z2, Z3 are accordingly set, such that the desired target values of Z1, Z2, Z3 are achieved again.

The target values (Z1, Z2, Z3) are controlled through the adaptation of the control variables (R1, R2, R3), here the current intensity I_(B) of the nozzle 30, and the flow rates of the primary and/or secondary gases in H₂, in A_(r) at the nozzle 30, by virtue of which the target parameters Z1, Z2, Z3 can be set in a targeted manner.

Primary gases are argon (Ar) and/or helium (He) and secondary gas is, for example, hydrogen (H₂), these flowing through the nozzle 30.

Use may be made of one, two or three control variables, proceeding from an optimum desired state for Z1, Z2, Z3 for the three control variables R1, R2, R3 used here.

Similarly, it is possible for the gas flow rates {dot over (m)}_(G) of argon {dot over (m)}_(Ar) (FIG. 8) and also those of hydrogen {dot over (m)}_(H2) (FIG. 9) at the nozzle 30 to be controlled, in order to achieve the desired results, in particular for the voltage U_(B).

The material flow rate {dot over (m)}^(M) of the material flow is in this case preferably not varied during the control.

As a result of this control, the layer structure, the layer thickness and the layer weight m_(c) (FIG. 6) of the blade or vane and also the porosity p (FIG. 7) remain constant over time t.

As a result of the current intensity I_(B) being controlled (FIG. 4), the power P is kept relatively constant (FIG. 5). This is then also identifiable from the constant values of the particle temperatures and the particle velocities V_(p) (not shown).

FIG. 10 shows a nozzle 30, in which argon (Ar) or helium (He) are introduced as primary gas and/or hydrogen (H₂) is introduced as secondary gas at one nozzle end 31, and at the other end 33 material (Mx,y) is added.

By virtue of the application of the voltage U_(B) between the electrode 36 and the nozzle 30, a high-energy arc generates a plasma, which forms the plasma flame.

FIG. 11 shows a perspective view of a rotor blade 120 or guide vane 130 of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plant for generating electricity, a steam turbine or a compressor.

The blade or vane 120, 130 has, in succession along the longitudinal axis 121, a securing region 400, an adjoining blade or vane platform 403 and a main blade or vane part 406 and a blade or vane tip 415.

As a guide vane 130, the vane 130 may have a further platform (not shown) at its vane tip 415.

A blade or vane root 183, which is used to secure the rotor blades 120, 130 to a shaft or a disk (not shown), is formed in the securing region 400.

The blade or vane root 183 is designed, for example, in hammerhead form. Other configurations, such as a fir-tree or dovetail root, are possible.

The blade or vane 120, 130 has a leading edge 409 and a trailing edge 412 for a medium which flows past the main blade or vane part 406.

In the case of conventional blades or vanes 120, 130, by way of example solid metallic materials, in particular superalloys, are used in all regions 400, 403, 406 of the blade or vane 120, 130.

Superalloys of this type are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blade or vane 120, 130 may in this case be produced by a casting process, by means of directional solidification, by a forging process, by a milling process or combinations thereof.

Workpieces with a single-crystal structure or structures are used as components for machines which, in operation, are exposed to high mechanical, thermal and/or chemical stresses. Single-crystal workpieces of this type are produced, for example, by directional solidification from the melt. This involves casting processes in which the liquid metallic alloy solidifies to form the single-crystal structure, i.e. the single-crystal workpiece, or solidifies directionally.

In this case, dendritic crystals are oriented along the direction of heat flow and form either a columnar crystalline grain structure (i.e. grains which run over the entire length of the workpiece and are referred to here, in accordance with the language customarily used, as directionally solidified) or a single-crystal structure, i.e. the entire workpiece consists of one single crystal. In these processes, a transition to globular (polycrystalline) solidification needs to be avoided, since non-directional growth inevitably forms transverse and longitudinal grain boundaries, which negate the favorable properties of the directionally solidified or single-crystal component.

Where the text refers in general terms to directionally solidified microstructures, this is to be understood as meaning both single crystals, which do not have any grain boundaries or at most have small-angle grain boundaries, and columnar crystal structures, which do have grain boundaries running in the longitudinal direction but do not have any transverse grain boundaries. This second form of crystalline structures is also described as directionally solidified microstructures (directionally solidified structures).

Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0 892 090 A1.

The blades or vanes 120, 130 may likewise have coatings protecting against corrosion or oxidation, e.g. (MCrAlX; M is at least one element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon and/or at least one rare earth element, or hafnium (Hf)). Alloys of this type are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

The density is preferably 95% of the theoretical density.

A protective aluminum oxide layer (TGO=thermally grown oxide layer) is formed on the MCrAlX layer (as an intermediate layer or as the outermost layer).

The layer preferably has a composition Co-30Ni-28Cr-8Al-0.6Y-0.7Si or Co-28Ni-24Cr-10Al-0.6Y. In addition to these cobalt-based protective coatings, it is also preferable to use nickel-based protective layers, such as Ni-10Cr-12Al-0.6Y-3Re or Ni-12Co-21Cr-11Al-0.4Y-2Re or Ni-25Co-17Cr-10Al-0.4Y-1.5Re.

It is also possible for a thermal barrier coating, which is preferably the outermost layer and consists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide, to be present on the MCrAlX.

The thermal barrier coating covers the entire MCrAlX layer. Columnar grains are produced in the thermal barrier coating by suitable coating processes, such as for example electron beam physical vapor deposition (EB-PVD).

Other coating processes are possible, e.g. atmospheric plasma spraying (APS), LPPS, VPS or CVD. The thermal barrier coating may include grains that are porous or have micro-cracks or macro-cracks, in order to improve the resistance to thermal shocks. The thermal barrier coating is therefore preferably more porous than the MCrAlX layer.

Refurbishment means that, after they have been used, protective layers may have to be removed from components 120, 130 (e.g. by sand-blasting). Then, the corrosion and/or oxidation layers and products are removed. If appropriate, cracks in the component 120, 130 are also repaired. This is followed by recoating of the component 120, 130, after which the component 120, 130 can be reused.

The blade 120, 130 may be hollow or solid in form. If the blade 120, 130 is to be cooled, it is hollow and may also have film-cooling holes 418 (indicated by dashed lines). 

1. A method for thermal coating by means of a material flow (42) by means of a nozzle (30), in particular by means of a powder flow, in which a material (M_(xy)) of the material flow (42) is heated, partially melted and/or melted, in particular by means of a plasma or a flame, in which at least one of the target variables (Z₁, Z₂, Z₃, . . . ) material flow velocity (v_(p)) of the material flow (42) and/or temperature (T) of the material flow (42) and/or voltage (U_(B)) between an electrode (36) and the nozzle (30) are measured or determined and controlled.
 2. The method as claimed in claim 1, in which, as target variables (Z₁, Zd₂), the material flow velocity (v_(p)) and the voltage (U_(B)) between the nozzle (30) and the electrode (36) are controlled.
 3. The method as claimed in claim 1, in which, as target variables (Z₁, Zd₂), the temperature (T) of the material flow (42) and the material flow velocity (v_(p)) are controlled.
 4. The method as claimed in claim 1, in which, as target variables (Z₁, Z₂), the temperature (T) and the voltage (U_(B)) between the nozzle (30) and the electrode (36) are controlled.
 5. The method as claimed in claim 1, in which, as target variables (Z₁, Z₂, Z₃), the temperature (T) of the material flow (42), the material flow velocity (v_(p)) and the voltage (U_(B)) between the nozzle (30) and the electrode (36) are controlled.
 6. The method as claimed in one or more of claims 1, 2, 3, 4 and 5 in which the current intensity (I_(B)) between the nozzle (30) and the electrode (36) and/or the gas flow rates ({dot over (m)}_(H2), {dot over (m)}_(Ar)) of the nozzle (30) are varied as control variables (R1, R2, R3), in order to keep the target variables (Z1, Z2, Z3) in a specific tolerant range or constant.
 7. The method as claimed in one or more of claims 1 to 6, in which the current intensity (I_(B)) is increased or lowered as a control variable (R1, R2, R3).
 8. The method as claimed in one or more of claims 1 to 7, in which the gas flow rate ({dot over (m)}_(Ar), {dot over (m)}_(H2)) of the primary gases (argon, helium) and/or of the secondary gases (hydrogen, . . . ) of the nozzle (30) are increased or lowered as at least one control variable (R1, R2, R3).
 9. The method as claimed in one or more of claims 1 to 8, in which an HVOF method is used.
 10. The method as claimed in one or more of claims 1 to 9, in which a plasma spraying method is used. 